Initiating the pulling of a crystalline body from a molten stock

ABSTRACT

A METHOD AND APPARATUS FOR PULLING A CRYSTALLINE BODY BY A SEED CRYSTAL FASTENED IN A HOLDER FROM A MELTING STOCK HEATED, IN A COLD COIL CRUCIBLE, AT LEAST TO MELTING TEMPERATURE. THE METHOD COMPRISES HEATING THE HOLDER AND ONE END OF THE SEED CRYSTAL, MOVING THE PREHEATED ZONE TO THE OTHER END OF THE SEED CRYSTAL, APPROACHING THE HEATED END TO THE STOCK TO BE MOLTEN, WHEREBY THE LATTER IS HEATED TO A TEMPERATURE AT WHICH IT IS CONDUCTIVE, AND THEREAFTER, HEATING THE STOCK BY THE FIELD ISSUING FROM THE COLD COIL CRUCIBLE.

Sept. 18, 1973 T. RUMMEL 3,759,670

INITIATING THE PULLING OF A CRYSTALLINE BODY FROM A MOLTEN STOCK FiledSept 18. 1970 United States Patent 3,759,670 INITIATING THE PULLING OF ACRYSTALLINE BODY FROM A MOLTEN STOCK Theodor Rnmmel, Hannover-Kirchrode,Germany, as-

signor to Siemens Aktiengesellschaft, Berlin and Munich, Germany FiledSept. 18, 1970, Ser. No. 73,304 Claims priority, application Germany,Sept. 24, 1969, P 19 48 194.8 Int. Cl. B0lj 17/18 US. Cl. 23-301 SP 5Claims ABSTRACT OF THE DISCLOSURE The invention relates to a method forpulling a crystalline body by a seed crystal fastened in a holder, froma melting stock heated, in a cold coil crucible, at least to meltingtemperature and to a device for performing said method.

The periodical Laboratory Methods, vol. LXVII, No. 404, June 1963, pages301-307, describe a method for pulling a rod, for example silicon, froma molten liquid situated in a basket type coil crucible. This coilcrucible consists of metal wire, such as for example, copper or silver,where in channels are located which are passed by a coolant, e.g. water.The outer wall of this cold coil crucible is surrounded by the windingof a primary coil. The windings are positioned in planes perpendicularlyto the axis, through the crucible opening.

This primary coil is charged with high frequency AC, which inducessecondary currents in the cold coil crucible. These secondary currentsinduce secondary currents of their own in the melting stock which islocated in the coil crucible and which can thereby be heated to meltingtemperatures and above. One end of a second crystal, afiixed at theother end within a holder, is brought into contact with the meltingstock. The seed crystal is then.

withdrawn from the molten liquid and a monocrystalline rod, growing atthe seed crystal, is pulled from the molten liquid.

The pulling of a monocrystalline body from a molten liquid located in acold coil crucible has the advantage that the crystallized material hasvirtually no dislocations. Also, no undesirable impurities reach themelt liquid from the material of the cold coil crucible which is notwetted by the molten mass, and thus cannot penetrate into thecrystallized rod.

If the melting stock in the cold coil crucible is a poorly conductingcrystalline material, in its cold state for example, a semiconductormaterial such as pure silicon, this material must be preheated before itcan be heated by the field, issuing from the cold coil crucible. It isknown to preheat the stock by irradiating the same with a focusedinfrared radiation emitted by a projection lamp. During thisirradiation, the primary coil which surrounds the cold coil crucible isstressed with high frequency alternating current. After the stockobtains a sufliciently high temperature, through irradiation, it iscoupled to the field issuing from the cold coil crucible, and can now befurther heated and melted, without the projection lamp.

However, it takes a considerable time to heat, particularly, a largeamount of stock to be melted by a projection lamp, i.e. to a temperaturewhich is high enough for the coupling process. The time required to thisend is particularly protracted if the projection lamp is situatedoutside a recipient which contains the cold coil crucible with thecrystal seed and if the radiation it emits must, for example, penetratea quartz disc.

The invention has as its object to remedy this condition so as to heatthe largest possible part of the stock to be melted, situated in thecold coil crucible by the field which issues from the coil crucible,prior to melting said stock so that the process is considerablyaccelerated.

The objective is achieved according to the invention, by heating theseed crystal at one end by a heating device, approaching the heated endof the seed crystal to the stock to be melted, whereby the latter may beheated and that thereupon heating the stock by the field, issuing fromthe cold coil crucible.

It is preferable to have the heated end of the crystal seed brought intocontact with the melting stock. It is also advantageous to heat the seedcrystal at one end, at least to melting temperature.

Since the crystal seed may also consist of a material which is of poorconductivity at low temperatures, a device is provided for heating theseed crystal. This device comprises an induction coil and a preheatingbody which may be coupled to the field of the induction coil and whichis is a readily conducting material.

This type of heating device may preheat the seed crystal with the aid ofthe preheating body, for example, at the end which is clamped into theholder. Thereafter, an incandescence zone, which is heated by theinduction coil, is moved, starting at the preheated end, through arelative movement between the seed crystal and the induction coil, tothe free end of the seed crystal which is then brought close to themelting stock or brought into contact with the melting stock.

The invention and its advantages will be discussed in greater detailwith reference to the drawing, which shows the device.

The basket-type silver crucible 2, shown in the figure, comprises twohalf-rings 3a and 3b, with tubes 4:: and 4b placed at their bottom,which run together at their ends. These tubes 4a and 4b areinterconnected at their bottom end through a central portion 5, withchannels which run together and to which tubes 4a and 4b are attached.Within the rings halves 3a and 3b are hollow spaces, within which thehollow spaces of the tubes 4a and 4b, end.

The basket-type silver crucible 2 is suspended, within a recipient, notshown in the drawing, which is either evacuated or filled withprotective gas, from two hollow tubes 6 and 7. The hollow spaces ofthese tubes end at the hollow spaces of the ring halves 3a and 3b.Cooling water flows through tube 6, thence into the basket-type crucible2 and leaves the same through the other tube 7, after having traversedring half 3a, the tubes 4a, the central portion 5, the tubes 4b and thering half 3b.

The outer wall of the basket-type crucible 2 is enclosed by a primarycoil 8, which is also situated in the notillustrated recipient and isconnected to a high frequency generator, not shown.

The basket-shaped silver crucible 2, contains as the melting stock, apiece of solid, highly pure silicon 9, which attains this highelectrical conductivity only at a temperature of about 600 C. so thatthe electromagnetic field issuing from the silver crucible 2 and inducedthrough primary coil 8, becomes coupled to the silicon piece 9.

Above the opening of the silver crucible 2, is holder 10 on shaft 19,wherein an elongated seed crystal 11 of silicon is affixed. Also, abovethe opening of the basket-type crucible 2 is another shaft 12 at whoselower end is arranged an induction coil 13 traversed by a coolant, forexample, water. The shafts 19 and 12 are displaceable in axialdirection, as indicated by the double arrow. The holder consists ofelectricity conducting steel.

Prior to the pulling process, the seed crystal 11 attached in the holder10, is pushed through the induction coil 13 and the induction coil 13 isbrought so close to the holder 10, that the coil field becomes coupledto holder 10. Thereupon, the induction coil 13 is stressed with highfrequency alternating current, and the holder 10 and thus also the endof the crystal seed 11 clamped thereinto, are heated to a temperature ofabout 600 C. Thus. a preheated zone of increased electrical conductivitydevelops in the seed crystal 11, in the vicinity of the holder 10. Thepreheated zone is coupled to the field issuing from the induction coil13. A relative movement between the coil 13 and the seed crystal 11,causes the preheated zone of increased electrical conductivity in theseed crystal 11, to move toward the end of the seed crystal. Theinduction coil 13 is so strongly Stressed with energy that the lower endof the seed crystal 11 melts. The coil 13 and the seed crystal 11 arenow moved so far downward until the molten drop contacts the to bemolten stock 9, consisting of cold silicon. As a result, a region ofseveral cubic centimeters is heated in the stock 9, to a temperature ofat least 600 C. In this range, the field issuing from the baskettypesilver crucible 2, becomes coupled, so that gradually, the entire stock9 is molten through the energy, supplied to primary coil 8, and isheated thereby. Thereupon, the current supply to the induction coil 13is interrupted and the induction coil 13 is shifted axially to theholder 10. Subsequently, the holder 10 is removed together with the coil13, axially upward, and a rod-shaped crystal growing at the lower end ofthe seed crystal 11, is pulled from the molten silicon, in thebasket-shaped silver crucible 2.

Under certain circumstances, it is sufficient to heat the lower end ofthe seed crystal 1], only to glowing temperature. Also, the energysupply to the induction coil 13 may be disconnected immediately afterthe crystal seed 11 and the solid molten stock 9, are brought intocontact. Finally, a separate preheating device which may be coupled tothe induction coil 13, may be located beneath the holder 10, such as amolybdenum ring across which the induction coil 13 may be so pushed thatthe ring will not cause interturn short circuit.

The silicon, in the silver crucible 2, may weigh several hundred gramswhile the monocrystalline silicon seed crystal 11 has a diameter of 4mm. The power of the induction coil 13 is, in this case, about 2 kv. Thefrequency of the high frequency AC current, with which the primary coil8 is charged, is within the range of 0.5 to 1 mHz., while the output ofthe coil 8 is about 60 kw.

Although the crucible 2 is specifically described as being silver, itcan also be copper, as previously mentioned.

What is claimed is:

1. A method for pulling a crystalline body by a seed crystal fastened ina holder from a melting stock heated, in a cooled, multiwindinginduction heating coil crucible, at least to melting temperature, whichcomprises heating inductively, said holder and adjacent end of the seedcrystal by an inductive heating device, utilizing said holder tofacilitate developing a preheated zone of increased electricalconductivity in said heated end of said seed crystal, providing relativemovement between said inductive heating device and said seed crystal,moving said preheated zone of increased electrical conductivity towardthe other end of said seed crystal, approaching the heated end to thestock to be molten, whereby heat from the seed crystal heats the stockto a temperature at which said stock is conductive, and thereafterheating said stock by the field issuing from the cooled multiwindingcoil crucible.

2. The method of claim 1, wherein the heated end of the seed crystal isbrought in contact with the stock to be melted.

3. The method of cliam 1, wherein the one end of the seed crystal isheated at least to melting temperature.

4. The method of claim 1, wherein an induction coil and a preheatingbody which may be coupled to the field of the induction coil and is areadily conducting material are provided for heating the seed crystal.

5. Apparatus for pulling a crystalline body from a melting stock whichcomprises a cooled, multiwinding induction heating coil crucible, aninduction coil surrounding said crucible, a seed crystal holder abovethe opening of said crucible, a seed crystal in said seed crystalholder, a movable induction coil surrounding said seed crystal andinitially disposed about the end of said seed crystal adjacent to saidholder whereby said moveable induction coil heats said holder and apreheated zone of increased electrical conductivity is developed in theend of said seed crystal adjacent to said holder, said moveableinduction coil being moveable relative to said seed crystal so that saidpreheated zone of increased electrical conductivity is first moved tothe other end of said seed crystal to melt said other end of said seedcrystal and means to subsequently approach the said other end of theseed crystal with the stock in the crucible to be molten.

References Cited UNITED STATES PATENTS 2,664,496 12/1953 Brace 233012,793,103 5/1957 Emeis 23301 3,069,244 12/1962 Sterling 23-301 3,086,8504/1963 Marino et a]. 23301 3,090,673 5/1963 Sterling 23-30l 3,172,7343/1965 Narren 23301 3,258,314 6/1966 Redmond 23301 3,494,804 2/1970Hanks et a1. 23301 3,505,025 4/1970 Dessauer 23273 FOREIGN PATENTS230,742 10/1960 Australia 23273 233,354 4/1961 Australia 23301 1,108,5434/1960 France 23-301 1,289,950 2/1969 Germany 23301 1,326,841 4/1963France 23301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, AssistantExaminer U.S. Cl. X.R.

